Pathophysiology

Local wound infections account for approximately 95% of complications following a dog, cat, or human bite.30 Infections caused by Pasteurella species occur shortly after the bite, typically developing within 24 hours. Infections presenting more than 24 hours after the bite are more likely caused by Staphylococcus and Streptococcus species.31,32

Cellulitis and abscess formation are the most commonly reported infectious complications, although bites may damage deep structures, including arteries, nerves, tendons, joints, and bones. Osteomyelitis, tenosynovitis, and septic arthritis occur, especially with hand bites. Sepsis, disseminated intravascular coagulation, meningitis, endocarditis, and tetanus can develop after animal bites, and transmission of HIV and hepatitis B and C is possible from human bites.33-39

Puncture wounds, perhaps because of their deep inoculation of bacteria into the tissue, are at higher risk for infection as compared with other types of bites. One study of mammalian bites found a 30% infection rate for punctures, 15% for lacerations, and 0% for superficial wounds such as abrasions.31 Several studies have evaluated specific risk factors for infection following a bite. Bites to the hand and upper extremity, full-thickness wounds, wounds requiring debridement, and bites in patients older than 50 years are at increased risk for infection.14,40-42 (See Table 2.) Bites causing damage to deep structures such as bones, joints, tendons, and neurovascular bundles are also at high risk for infection. Bites to the head and neck, presumably because of their excellent blood supply, are considered low risk; three studies of dog bites found infection rates of 1.4%- 4.0% for bites to the head and neck.14,40,43

“I loathe people who keep dogs. They are the cowards who haven’t got the guts to bite people themselves.”—August Strindberg

Dog bites have reported infection rates of 1.6%-30.0%, with the highest rates of infection occurring in the hand.4,10,14,17,31,40- 42,45-52 Talan et al found the median interval between the time of the bite and presentation to be 35 hours, and that only 12% of the infections presented as abscesses. Furthermore, the wounds averaged five different isolates per wound culture, with approximately half being mixed aerobic/ anaerobic infections.32 Two recent studies show Pasteurella species to be the most common organism present in dog bite infections, found in 50%-53% of the infected wounds.14,32 These findings were significantly different from older studies, which reported Pasteurella species as relatively uncommon in dog bites.31,50,53 Staphylococcus and Streptococcus species are also frequent pathogens, present in 24%-46% and 29%-46% of infections, respectively.14,32

Damage to deep structures can occur from any dog ite, but it is more likely in cases involving a law enforcement dog. These dogs typically bite with greater force than civilian dogs and practice a bite-and-hold technique.54,55 One study of law enforcement dog bites found vascular injuries present in 6.9%, open fractures in 2.4%, nerve injuries in 1.7%, and tendon lacerations in 1.2%.56

Facial bone fractures from dog bites have been described, mostly in children.57,58 The periorbital and nasal bones are commonly involved.58 While rare, skull fractures with underlying brain injury, as well as dural penetration with resultant meningitis and brain abscess, can occur in small children bitten by large dogs.59-62

Cat Bites

Reported wound infection rates from cat bites are high, ranging from 15.6% to 50%.18,19,63 This increased rate of infection is multifactorial; the higher incidence of puncture wounds, a higher proportion of hand bites, an older average age for cat bite victims, and a higher likelihood of Pasteurella species in the feline flora all play a role. The median interval between the time of the bite and presentation is generally shorter for cat bites (23 hours) than for dog bites (35 hours),32 which is probably due to the greater likelihood of Pasteurella species in cat bite infections.16,32

Cat bite wound infections average approximately six different isolates per culture, with the majority (63%) of infections being mixed aerobic/anaerobic. Approximately one-fifth of cat bite infections are abscesses.32 Pasteurella species are the most common organisms, present in 57%-75%, but Streptococcus and Staphylococcus species are also common pathogens, present in 46% and 35%, respectively.18,32

Human Bites

“…of all bites, that of the human animal is the worst.”—F.K. Boland

While this statement might not be true for all human bites, it does appear accurate for those caused by CFIs. Human bites have reported infection rates of 3%-50%.2,23,27,31,64 Most studies of human bite wounds include a high percentage of patients with CFIs, which have notoriously high rates of infection. Studies evaluating human bites in children have shown generally lower rates of infection (3.4%-9.3%) as compared with studies in adults.22,65

Human occlusional bites generally cause less tearing and crush injury than dog bites. One interesting study compared the rate of infection of 434 occlusional bites to that of 803 simple lacerations in the same group of institutionalized patients. The occlusional bites had an infection rate of 17.7%, only slightly higher than the simple lacerations (13.4%). However, a higher percentage (14%) of the human bites received prophylactic antibiotics than did the lacerations (3.2%) in this study.66 Some authors have concluded that human bites to locations other than the hand are at no greater risk for infection than other mammalian bites.4,30

Human bites to the hand, and specifically CFIs, are at increased risk for infection. Up to one-third of hand infections following human bites involve deep structures (tendon sheath, bone, or joint).67 The high rate of deep structure infection in CFIs appears to be due to several factors: the frequent penetration of the metacarpophalangeal joint and tendon sheaths (which are covered by a relatively thin layer of skin), the high incidence of associated open fractures, and delay in seeking medical care. Other contributing factors may include deceptive stories regarding mechanism of injury, noncompliance, and because the typical CFI wound is small (usually 3-8 mm) and difficult to explore without extending the wound. Furthermore, injuries of the extensor tendon sheath incurred while the hand is clenched will retract proximally when the fingers are extended, making identification and appropriate wound care of these injuries difficult. While occlusional human bites may occasionally cause avulsion and amputation type injuries, CFIs have consistently high rates of deep structure damage. Damage to underlying bones occurs in 17%-58% of CFIs; 52%-62% violate a joint capsule (typically the metacarpophalangeal joint), and 15%-20% have associated tendon injury.28,68 Given these facts, it’s not surprising that CFIs have alarmingly high rates of osteomyelitis (16%), septic arthritis (12%), and tenosynovitis (22%).27

Human saliva has, on average, a concentration of 108 organisms per mL, with 42 different bacterial species present.69 Thus, infections resulting from human bites are typically polymicrobial, with an average of five different organisms isolated per wound. Streptococcus species are present in 60%-80% of infected human bites, Staphylococcus aureus in 37%-46%, and anaerobic species in 43%-60%.68,70-72 Anaerobes become more prevalent in infected human bites as the time interval from injury to treatment increases.73 Eikenella corrodens, a fastidious, facultative anaerobe, is present in 20%-25% of infected human bites. E. corrodens is resistant to multiple antibiotics, including clindamycin, erythromycin, and the first-generation cephalosporins, and is felt to be at least partially responsible for the high morbidity of human bite wounds.70-72

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